1. Field of the Invention
This invention relates to data coding.
2. Description of the Prior Art
Channel coding is employed in digital magnetic recording apparatus such as digital video tape recorders (DVTRs) in order to match the frequency spectrum of data to be recorded to the frequency response of the recording medium. For example, a previously proposed DVTR employs a channel code (a so-called 8:16 code) to map 8-bit data words onto 16-bit code words before the code words are recorded onto a magnetic tape medium. The code words generated by the channel code obey a number of rules relating to the maximum runlength of a single binary value (to control the low frequency content of the coded data), the minimum runlength of a single binary value (to control the high frequency content of the coded data) and the balance between ones and zeroes (to control the dc content of the coded data).
In the 8:16 code, each of the 256 possible 8-bit data words has two complementary corresponding 16-bit codes for recording on the tape medium. This means that there are 512 valid 16-bit codes out of a possible 2.sup.16 (65536) permutations. In the previously proposed DVTR mentioned above, if a 16-bit replayed group does not correspond to one of the 512 valid codes (for example due to a replay data error), then that code is simply flagged as being erroneous.